Fiber optic communication systems use glass fibers as optical waveguides. Information is encoded into a modulated light signal that is propagated along the fibers. Propagation losses in the optical fibers necessitate the use of amplifiers if long distance communication is required. Originally, the optical signal was amplified by converting it to an electrical signal, amplifying it and then converting it back to an optical signal. More recently, erbium-doped fiber amplifiers (EDFAs) have been used. EDFAs are all-optical amplifiers, but have limited bandwidths, high noise levels and require a special type of fiber. Raman amplifiers are an alternative to EDFAs that address all of these problems. They provide a large gain over a wide bandwidth while maintaining small noise figures, and they can be made using standard silica fibers.
Raman amplification occurs when lightwaves from a high energy “pump” laser interact with the crystalline lattice of the optical fiber. The atom absorbs the light, then re-emits a photon with energy equal to the original photon plus or minus the atomic vibration. If the excitation occurs simultaneously with the interaction of another “communication” photon within the proper frequency range, an identical “twin” photon will be emitted in phase with the communication photon. The waves of potential energy created by the absorption of the pump photons are called Phonons. Silica glass fibers support a wide range of phonon frequencies, which allows for a very wide Raman gain bandwidth.
In a Raman amplifier, a high-power pump beam is propagated down a fiber simultaneously with the signal. The pump-beam is absorbed in the silica fibers in a virtual reservoir of signal energy, in the form of acoustic phonons. Since the signal and the pump propagate in the same fiber, the signal acts to stimulate emission of photons with the same modulation and frequency as itself.
One problem with Raman amplifiers is the potential for noise introduced by double Rayleigh backscatter. Rayleigh scattering is caused by refractive index inhomogeneities in the fiber that are small compared to the wavelength. It has been shown that the double Rayleigh backscatter (DRB) is the same spectral frequency as the primary signal and hence it is also referred to In-band Crosstalk (IBX). IBX is indistinguishable from the signal in the wavelength (frequency) domain, because it consists of signal photons that have been multiply-delayed before reaching the receiver, resulting in background noise in the time domain, rather than the wavelength domain. For most optical amplifiers the effect of double Rayleigh backscatter is negligible. However, in a Raman amplifier, the amplification is distributed and takes place over a significant length of fiber. Hence, there is an increased possibility of double Rayleigh backscatter. Additionally, since the amplification medium is also the scattering medium, the double Rayleigh backscatter is amplified on each journey through the amplifier, while the signal only benefits from a single journey.
Double Rayleigh backscatter is very difficult to measure using optical instrumentation and continuous source test methods, such as the Interpolation with Source Subtraction (ISS) method.
A prior attempt to measure double Rayleigh backscatter is described in ‘Characterization of Double Rayleigh Scatter Noise in Raman Amplifiers’, Lewis, Chermikov and Taylor, IEE Photonic Technology Letters, Vol. 12, No. 5, May 2000. Lewis et al., use a signal modulated through a high-extinction ratio Acousto-Optic Modulator (AOM) on the amplifier input. The modulation rate is assumed to be high enough to avoid modulation of the double backscattered signal at the receiver end. At the output, a second Acousto-Optic Modulator runs 180° out-of-phase with the input AOM. The output from the second AOM is collected by an optical spectrum analyzer. The amplified spontaneous emission (ASE) is measured using interpolated measurements from either side of the signal band, and the combination of ASE and double backscatter are measured in band. The difference between the two measured quantities yields the double backscatter. This method requires complete source signal extinction to work accurately.
The use of out-of-phase AOMs is designed to extinguish the source signal and leave the noise on the output. However, it is difficult to completely extinguish the signal (source or output), particularly when the amplifier gain is high. This is a problem, for example, in a research and development environment, where a wide variety of amplifier types and properties are used.